When numerous observations can be _synoptically studied_, forecasting becomes much more nearly certain. For this purpose telegraphic communications are indispensable. The continent of Europe is better placed than England for accurate forecasting. Areas of high pressure coincide usually with large areas of land, of low pressure with large surfaces of water. Thus England is placed near the boundary of the usual anticyclonic and cyclonic systems, and its chief disturbances come from the Atlantic from which early communication is impracticable. Furthermore cyclonic disturbances may be diverted from their course by a coastline or mountains or by the formation of an anticyclonic area. In view of these uncertainties, the large proportion of correct forecasts is somewhat surprising. The =Moisture of the Air= depends upon the amount of vapour present in it, and the ratio of this to the amount which would saturate the air at the actual temperature. The former is called the _absolute humidity_, the latter the _relative humidity_. The _dew point_ is the point at which condensation of some of the vapour in the atmosphere occurs, either as dew, rain, snow, or hoar-frost. The amount of moisture which the atmosphere can retain before such condensation occurs, varies with the temperature (see page 101). Thus the air is drier at noon than at midnight, though the amount of vapour present in the two cases be the same; and it is for the most part drier in summer than in winter. This refers to the relative humidity, which is highest in cold weather. The absolute humidity is higher in summer than in winter; it varies more in continental than in maritime and insular climates; and there are daily variations according to the state of the sky, the movements of air, etc. The relative humidity is expressed as a percentage of what would be required to produce saturation at the given temperature. The usual relative humidity is 50 to 75 per cent. A moist air prevents excessive changes of temperature due to radiation. It protects the earth from too great intensity of the solar rays by day and from too rapid loss of heat by radiation at night. The inhalation of a dry air plays an important part in the cure of consumption. When the air is almost saturated with moisture, evaporation from the skin and lungs is diminished, and there is a feeling of oppression and disinclination to work caused by the interference with the tissue changes of the system. =Rainfall= is caused by over-saturation of a column of moist air. This may be due to the contact of the air with a cold surface, as the ridge of a mountain or a large surface of water, or to the impact of a colder wind. The amount of rainfall varies greatly. In some parts there is no rain, as in the desert of Sahara; while on the south-east slopes of the Himalayas, which are exposed to winds laden with moisture, it may be several hundred inches.